AME 23:163-175 (2001) - doi:10.3354/ame023163

Seasonal variations of photosynthetic ciliates at a Mediterranean coastal site

M. Modigh*

ABSTRACT: Standing stocks of ciliated protozoa were analyzed in weekly samples over 3 yr at a Mediterranean coastal site (Gulf of Naples, Tyrrhennian Sea). Photosynthetic ciliates contributed 49% of total ciliate biomass; heterotrophic naked choreotrichs
and tintinnids contributed 25 and 16%, respectively. The present paper focuses on the occurrence and succession of the autotrophic ciliate Mesodinium rubrum and of aloricate mixotrophic and heterotrophic choreotrichs. A very diverse assemblage of
Strombidium spp. dominated the mixotrophic choreotrich populations, while Tontonia spp. contributed 14.3% and Laboea strobila 4.7% to plastidic ciliate abundance. The annual pattern of occurrence of these 3 genera of mixotrophic
ciliates was similar over the 3 years of this study. Autotrophic and mixotrophic ciliates differed in the periods of maximum relative contribution to the ciliate assemblage. Mixotrophic choreotrichs dominated the ciliate populations in spring and summer
while the maximum contribution of autotrophic ciliates occurred in winter. M. rubrum occurred in 2 forms‹the larger form was the only Mesodinium observed in winter, while it was absent during summer, when the smaller Mesodinium was at
times observed. The contribution of M. rubrum to annual primary production in the Gulf of Naples was about 3%. Aloricate choreotrich populations were dominated by small cells; 92% of the aloricate heterotrophs were <30 μm equivalent spherical
diameter (ESD), and for the mixotrophic ciliates 73% were <30 μm ESD and 24% were between 30 and 50 μm ESD. Notwithstanding a very wide range of variation in aloricate choreotrich densities, 135 to 48800 cells l-1, the relative contribution
of different size classes was remarkably stable at any level of abundance. The calculated production for aloricate choreotrichs was between 2.3 and 3.3 g C m-3 yr-1 for the surface layer, about 50% of which was estimated to rely on
ciliate grazing of bacterio- and picoplankton, as well as on photosynthesis covering, in part, the carbon demand of mixotrophic ciliates.